This invention relates generally to hard computer memory discs, and more particularly concerns a method for machining the memory disc's thin aluminum substrate to assure that the resulting memory disc has a high degree of flatness.
A computer memory disc stores computer information on the magnetic coating on the surface of the memory disc. The information stored on the disc is read off the disc by a read head that is indexed across the disc as the disc spins at a high rate of speed. In order to pack the most information in the smallest amount of space on the disc, it is necessary to have the read head track as close to the disc surface as possible. In accordance with standard practice, the read head literally floats above the disc on a thin film of air. Because the read head tracks so close to the disc surface, it is important that the disc be as flat as possible.
In accordance with prior art practices, computer memory discs are made by machining a very thin aluminum disc to a finished surface. The finished surface of the aluminum disc is then electroplated with nickel to fill in any gaps or holes in the surface. The nickel finish is polished and is then coated with a very thin layer of cobalt, which is the magnetic material on which the computer information is recorded. In addition to filling gaps in the aluminum surface, the nickel also provides an interface between the aluminum substrate and the cobalt to assure adhesion of the cobalt to the aluminum surface.
As an alternative to the nickel/cobalt disc configuration, the finished aluminum disc is coated with an epoxy material containing iron particles by spraying the epoxy material onto the spinning disc. The magnetic material is magnetically aligned while the epoxy is still wet, the disc is baked, and the coating is then polished flat.
There are several methods available for machining the aluminum substrate. Each method has its own disadvantages. One prior art method first uses a carbide or cemented diamond tool to rough turn the aluminum disc. The rough turning operation produces a spiral pattern similar to the pattern of the groove in an ordinary phonograph record. The rough turning, spiral cutting process produces stresses in the thin aluminum disc, which stresses cause the thin aluminum disc to assume a slightly concave/convex configuration called dishing. In order to eliminate the concave/convex configuration the rough finished aluminum discs are stacked between ground and lapped ceramic plates, loaded with between 25-100 psi of pressure, and inserted into a furnace to remove the concave/convex configuration. For aluminum discs that have been annealed, heat flattening can be accomplished at approximately 580.degree. F. Otherwise the temperature must exceed 650.degree. F. in order to anneal and thus flatten the disc.
The resulting heat flattened aluminum disc is then given a final finished by spiral cutting again. Even though the final turning removes only a very small amount of material, the final turning still produces sufficient spiral stresses to cause dishing of the aluminum disc. In order to eliminate the dishing in the finished aluminum disc, the disc is electroplated with a sufficiently thick layer of nickel to fill in the concave/convex configuration. The nickel plated disc must then be polished to remove the excess nickel to achieve the requisite flatness. The heat flattening, extra nickel coating, and extra polishing to remove the excess nickel, of course, add cost to the finished product.
Another method of producing the finished aluminum discs begins by double disc grinding the rough aluminum disc blanks. Double disc grinding in most cases produces a very flat part because the grinding takes place simultaneously on both sides of the aluminum disc. The difficulty with the double disc grinding is that sub-micron particles are buried in the inclusions of the aluminum surface. Moreover, the resulting surface is usually too coarse to simply be polished prior to coating with nickel. Usually double disc ground aluminum discs are ground to 0.077 inch thickness because random scratches created by the abrasives may be from 0.0008 to 0.0015 inches deep. Therefore, a substantial amount of material must be removed from the disc during the polishing process in order to eliminate the deep scratches. Polishing that amount of material, of course, is not cost effective, especially where a grain of abrasive from the rough double disc grinding may be freed from an inclusion and contaminate the diamond polishing pads. Therefore, it is usually necessary to undertake a second disc grinding with a finer abrasive prior to final polishing. The three step process, double disc grinding (rough), disc grinding (medium), and finish polishing, is not cost effective although the resulting finished aluminum disc is sufficiently flat to be economically coated with nickel and polished before being coated with cobalt.
Another method of machining the aluminum disc begins with the same double disc grinding process to remove the rough material. The double disc ground aluminum discs are then turned in a spiral pattern on a lathe to remove 0.001 to 0.0015 inches of material to provide the final finish. Although the final turning process removes a very small amount of material, the parallel spiral stresses still result in dishing of the aluminum disc. That dishing as previously described must be eliminated by coating an excess amount of nickel and then polishing off the extra nickel to bring the resulting nickel coated disc back to requisite flatness.